Decanting centrifuges include a bowl that is rotatably driven about a horizontal or vertical axis and contains a helical scroll, or worm-type, conveyor to separate a slurry fed into the bowl at a first end into its constituent solids and liquid(s). The conveyor rotates at a different speed within the bowl to scroll the heavier solids to discharge ports at a second end of the bowl. The separated liquid flows primarily in the opposite direction and is discharged from ports at the first end of the bowl. The decanting centrifuge can be of two general types, either solid bowl or screen bowl. In the latter, the solids are scrolled by the conveyor over an additional perforated screen section of the bowl prior to discharge.
Existing decanting centrifuges of both the solid and screen bowl types operate when fed with a slurry containing solids either to separate the solid particles from the liquid, or to classify the solids, that is to divide the solids so that particles above a certain size are discharged as solids and particles below that size are discharged with the liquid.
A limitation on the design and operation of large decanting centrifuges is the torque required to drive the conveyor. A number of factors contribute to the torque requirement, with the frictional effect of the solids being conveyed over the screen section being a major component of the required torque. One approach to reducing the torque requirement has been to provide the screen section of the bowl in a diverging frusto-conical form, considered in the directions towards the solids outlet, or discharge end. This arrangement reduces the conveyor torque requirements as the centrifugal forces on the solids assist passage of the solids along the diverging screen section. Reducing the conveyor torque requirement permits a reduction in the size and cost of the conveyor drive including, for example the gearbox, and/or a reduction in the total power consumption of the centrifuge. The use of the diverging screen section also provides a higher G factor and improves dewatering. Increasing feed rates also increase cake pile heights which have an adverse effect on product moisture. The diverging screen causes the cake pile height to become lower, resulting in better dewatering.